Magnetic memory

ABSTRACT

A magnetic memory according to an embodiment includes: a magnetic layer including a plurality of magnetic domains and a plurality of domain walls, and extending in a direction; a pinning layer formed with nonmagnetic phases and magnetic phases, extending in an extending direction of the magnetic layer and being located adjacent to the magnetic layer; an electrode layer located on the opposite side of the pinning layer from the magnetic layer; an insulating layer located between the pinning layer and the electrode layer; a current introducing unit flowing a shift current to the magnetic layer, the shift current causing the domain walls to shift; a write unit writing information into the magnetic layer; a read unit reading information from the magnetic layer; and a voltage generating unit generating a voltage to be applied between the pinning layer and the electrode layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2012-60414 filed on Mar. 16, 2012in Japan, the entire contents of which are incorporated herein byreference.

FIELD

Embodiments described herein relate generally to magnetic memories eachhaving a magnetic nanowire in which information is recorded in each ofmagnetic domains separated from one another by domain walls.

BACKGROUND

In recent years, there have been reports that domain wall shifts bycurrent were observed in magnetic nanowires with submicron widths.Magnetic memories that can cause domain walls to shift by takingadvantage of the reported effect have been suggested. In such a magneticmemory, a magnetic nanowire divided into plural magnetic domains isused, and information (data) “0” or “1” is associated with themagnetization directions of the magnetic domains. In this manner,information is stored. When a current is flowed, the domain walls shift,and accordingly, the magnetic domains shift. As a result, theinformation (data) stored in the magnetic domains also shifts, and canbe read by a sensor and be written by a write unit. That is, themagnetic domains are equivalent to memory cells. In general, there is anincreasing demand for magnetic memories that have larger capacitiesthrough increases in cell density or the like, and consume less power.

In a magnetic memory of the above described domain wall shifting type, apinning site for pinning the domain walls is necessary to preventchanges in the locations of the domain walls due to external disturbancesuch as heat. As a specific example, a method of forming a physicalnotch in a magnetic nanowire has been suggested. Also, a method offorming a domain wall pinning site by ion beam irradiation has beensuggested.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a magnetic memory according to afirst embodiment;

FIGS. 2( a) through 2(e) are schematic views illustrating forms ofpinning layers;

FIGS. 3( a) through 3(c) are diagrams for explaining the relationshipbetween the domain walls of the magnetic nanowire and the pinning layer;

FIG. 4 is a diagram for explaining application of an electric field tothe pinning layer;

FIGS. 5( a) and 5(b) are diagrams for explaining pinning potentialcontrol;

FIG. 6 is a diagram showing an example timing chart of a shiftoperation;

FIGS. 7( a) and 7(b) are diagrams for explaining a spin torque writemethod;

FIGS. 8( a) and 8(b) are diagrams for explaining write methods using amagnetic field generated by a current;

FIGS. 9( a) through 9(c) are diagrams for explaining read methods;

FIG. 10 is a top view of the magnetic memory according to a firstembodiment;

FIG. 11 is a cross-sectional view of a magnetic memory according to asecond embodiment;

FIG. 12 is a cross-sectional view of a magnetic memory according to athird embodiment;

FIGS. 13( a) through 13(d) are diagrams for explaining the pinningpotential generated from the pinning layer;

FIG. 14 is a top view of a magnetic memory according to Example 1;

FIGS. 15( a) through 15(c) are cross-sectional views illustratingprocedures for manufacturing the magnetic memory according to Example 1;

FIGS. 16( a) and 16(b) are cross-sectional views illustrating proceduresfor manufacturing the magnetic memory according to Example 1;

FIG. 17 is a top view of the pinning layer formed according to Example2;

FIGS. 18( a) through 18(d) are cross-sectional views illustrating theprocedures for manufacturing the pinning layer of Example 2;

FIG. 19 is a diagram showing an example of a timing chart using a spintorque write method; and

FIG. 20 is a diagram showing an example of a timing chart using a TMRread method.

DETAILED DESCRIPTION

A magnetic memory according to an embodiment includes: a magnetic layerincluding a plurality of magnetic domains and a plurality of domainwalls separating the magnetic domains from one another, the magneticlayer extending in a direction; a pinning layer formed with nonmagneticphases and magnetic phases, the pinning layer extending in an extendingdirection of the magnetic layer and being located adjacent to themagnetic layer; an electrode layer located on the opposite side of thepinning layer from the magnetic layer; an insulating layer locatedbetween the pinning layer and the electrode layer; a current introducingunit configured to flow a shift current to the magnetic layer, the shiftcurrent causing the domain walls to shift; a write unit configured towrite information into the magnetic layer; a read unit configured toread information from the magnetic layer; and a voltage generating unitconfigured to generate a voltage to be applied between the pinning layerand the electrode layer.

The following is a detailed description of embodiments, with referenceto the accompanying drawings.

First Embodiment

FIG. 1 shows the structure of a magnetic memory of a domain wallshifting type according to a first embodiment. The magnetic memory 1 ofthe first embodiment includes: a magnetic nanowire 10 that functions asa memory element located above a substrate 100 having an integratedcircuit (not shown) mounted thereon; a write unit 16 for writingmagnetic information into this magnetic nanowire 10; and a read unit 18for reading magnetic information. A pinning layer 20 for pinning domainwalls is located adjacent to the surface of the magnetic nanowire 10 onthe opposite side from the substrate 100. It should be noted that amagnetic nanowire means a magnetic layer that extends in one direction.An electrode layer 40 extending in the extending direction (thelongitudinal direction) of the magnetic nanowire 10 is provided on theopposite side of the pinning layer 20 from the magnetic nanowire 10, andan insulating layer 30 is provided between the magnetic nanowire 10 andthe electrode layer 40. Accordingly, the pinning layer 20, theinsulating layer 30, and the electrode layer 40 are provided in theextending direction of the magnetic nanowire 10. Also, a voltagegenerating unit 50 that generates a voltage between the magneticnanowire 10 and the electrode layer 40, and applies an electric field tothe pinning layer 20 is provided. The magnetic memory 1 of thisembodiment is provided on a substrate having an integrated circuitformed thereon, but may not be provided on a substrate.

The magnetization of the magnetic nanowire 10 has an easy axis ofmagnetization in a direction perpendicular to the plane formed by thelongitudinal direction of the magnetic nanowire 10 and the long side ofa cross-section perpendicular to the longitudinal direction of themagnetic nanowire 10 (perpendicular magnetic anisotropy). In a casewhere the cross-section of the magnetic nanowire 10 is square in shape,the long side thereof is any side of the square shape. In the magneticnanowire 10, magnetic domains 10 a and domain walls 10 b separatingthose magnetic domains 10 a are formed, and the directions of themagnetic moments of the magnetic domains 10 a are associated with data“1” or “0”, to record information. The cross-section of the magneticnanowire 10 is rectangular, square, elliptical, or circular in shape,for example. The thickness of the magnetic nanowire 10 shown in FIG. 1is uniform in the longitudinal direction, but can cyclically vary. Ashift current Is is flowed to the magnetic nanowire 10 from a currentsource via current introducing portions (not shown), so that thelocations of the domain walls 10 b are made to shift, and data are movedin the magnetic nanowire 10.

The pinning layer 20 provided adjacent to the magnetic nanowire 10 isformed with two types of phases existing in the plane of the pinninglayer 20. The two types of phases are a magnetic phase region and anonmagnetic phase region. Examples of two-phase structures of pinninglayers 20 are shown in FIGS. 2( a) through 2(e). FIGS. 2( a) through2(e) are schematic views of the respective pinning layers 20 viewed fromabove.

A first form of a pinning layer 20 has a structure in which magneticphases 24 are precipitated in a granular state in a nonmagnetic phasematrix 22, as shown in FIG. 2( a). This structure can be a material suchas CoCrPt—SiO₂. CoCrPt—SiO₂ is a material that can be separated into twophases even if formed by co-sputtering or the like. Further, in thestructure illustrated in FIG. 2( a), magnetic phases of a FePt alloy orthe like can be formed like islands in a nonmagnetic phase formed withvacuum. Alternatively, the islands of FePt can be surrounded by a MgOnonmagnetic phase. As shown in FIG. 2( b), a granular form can also berealized where nonmagnetic phases 22 are precipitated in the magneticphase matrix 24, which is the opposite of the example illustrated inFIG. 2( a). This granular form can also achieve a pinning effect.

A second form of a pinning layer 20 is formed with dot-like magneticphases 24 and a nonmagnetic phase 22 surrounding the dots, as shown inFIG. 2( c). As will be described later in Example 2, this pinning layer20 can be formed by using a directed self-assembly (DSA) of diblockcopolymer or the like. The dot pitch (dot diameter+interval betweendots) preferably corresponds to the size of one record bit, becausecontrolling memory operations would become easier. To achieve stable bitshifts, the dot diameter is preferably equal to each of the intervalsbetween the dots, or the dot diameter is preferably larger than each ofthe intervals between the dots.

In a third form of a pinning layer 20, magnetic phases 24 andnonmagnetic phases 22 are arranged in a striped form, as shown in FIGS.2( d) and 2(e). This structure can be formed by using a technique suchas DSA, as well as a top-down method such as EB lithography. The pitchof stripes (=the width of one stripe of the magnetic phases 24+the widthof one stripe of the nonmagnetic phases 22) is preferably equal to thesize of one bit (one magnetic domain) (FIG. 2( d)), because controllingmemory operations would become easier. Alternatively, the pitch ofstripes is made smaller than the size of one bit so that two or morestripes of the magnetic phases 24 correspond to one bit (FIG. 2( e)).This is preferable, because precise stripe location control would becomeunnecessary. In a case where the pitch of stripes is made equal to thesize of one bit, the width of each stripe of the magnetic phases 24 ispreferably equal to or smaller than the width of each stripe of thenonmagnetic phases 22, so as to achieve more stable bit shifts.

Next, the material of the magnetic nanowire 10, and the materials of themagnetic phase(s) 24 and the nonmagnetic phase(s) 22 of the pinninglayer 20 are described.

The magnetic nanowire 10 and the magnetic phase(s) 24 of the pinninglayer 20 can be formed with (1) a disordered alloy, (2) an orderedalloy, or (3) a ferrimagnetic material, for example.

Disordered Alloy

The disordered alloy can be a metal or an alloy containing at least oneelement selected from Fe, Co, and Ni. Examples of such disordered alloysinclude a NiFe alloy, a NiFeCo alloy, a CoFeB alloy, a CoCr alloy, aCoPt alloy, a CoCrTa alloy, a CoCrPt alloy, a CoCrPtTa alloy, and aCoCrNb alloy. Since those alloys have disordered structures or amorphousstructures, manufacturing becomes easier, and magnetic anisotropy energyand saturation magnetization can be controlled by adjusting theproportions of the elements.

Ordered Alloy

The ordered alloy can be an alloy formed with at least one elementselected from Fe, Co, and Ni, and at least one element selected from Ptand Pd. Examples of such ordered alloys having L₁₀ crystallinestructures include Co₅₀Pd₅₀, Co₅₀Pt₅₀, Fe₅₀Pt₅₀, Fe₅₀Pd₅₀, Fe₃₀Ni₂₀Pd₅₀,Co₃₀Fe₁₀Ni₁₀Pt₅₀, and Co₃₀Ni₂₀Pt₅₀. Those ordered alloys are not limitedby the above composition ratios. Magnetic anisotropy energy andsaturation magnetization can be adjusted by adding an impurity elementsuch as Cu, Cr, or Ag to those ordered alloys. In this manner, largemagnetic anisotropy energy can be easily achieved.

Ferrimagnetic Material

The ferrimagnetic material can be an alloy of a rare-earth metal and atransition metal. Examples of such ferrimagnetic materials include anamorphous alloy formed with at least one element selected from Tb(terbium), Dy (dysprosium), and Gd (gadolinium), and at least oneelement selected from transition metals such as Fe, Co, and Ni. Morespecific examples include TbFe, TbCo, TbFeCo, DyTbFeCo, GdTbCo, andGdFeCo. Magnetic anisotropy energy and saturation magnetization can becontrolled by adjusting the compositions of those alloys. Thoseamorphous alloys can contain fine crystals. As the material of themagnetic nanowire 10, a ferrimagnetic material that can achieve lowsaturation magnetization is preferable. As saturation magnetization ismade lower, the current amount required for domain wall shifts can bemade smaller.

A combination of the materials (1) through (3), or a combination such asTbFeCo/NiFe or GdFeCo/CoFeB, can be used as the magnetic nanowire 10, toadjust the domain wall characteristics. Also, it is possible to use astack of ferrimagnetic layers with different compositions. Lowerapparent saturation magnetization can be achieved by stacking arare-earth-rich amorphous alloy of a rare-earth metal and a transitionmetal, and a transition-metal-rich amorphous alloy of a rare-earth metaland a transition metal. Accordingly, such a film stack is used as thematerial of the magnetic nanowire 10, to reduce the current amountrequired for shifting the domain walls 10 b.

The material of the magnetic nanowire 10 and the material of themagnetic phase(s) 24 of the pinning layer 20 do not need to be differentfrom each other, and should be chosen in accordance with desired designdetails. The material of the magnetic nanowire 10 can be the same as thematerial of the magnetic phase(s) 24 of the pinning layer 20.

Other than the above described conductive magnetic phase(s), an oxidecontaining a 3d transition metal serving as a semiconductor or aninsulator can be used as the magnetic phase(s) 24 of the pinning layer20.

As the nonmagnetic phase(s) 22 of the pinning layer 20, nonmagneticmetal phase(s) and/or nonmagnetic insulating phase(s) can be used.Examples of such nonmagnetic metal phases include Pt, Au, Ag, Cu, Al, oran alloy containing at least one of those elements. Examples ofnonmagnetic insulating phases include insulating materials such asaluminum oxide (AlOx), aluminum nitride (AlN), magnesium oxide (MgO),magnesium nitride (Mg—N), silicon oxide (SiO₂), silicon nitride (Si—N),silicon oxynitride (Si—O—N), TiO₂, and Cr₂O₃. Other than those, it ispossible to use a dielectric material such as barium titanate (BaTiO₃),SrTiO₃, PbTiO₃, or HfO₂.

With such a pinning layer 20 located adjacent to the magnetic nanowire10, the potential of the domain walls 10 b is lower in a case where thedomain walls 10 b are in contact with the nonmagnetic phase(s) 22 thanin a case where the domain walls 10 b are in contact with the magneticphase(s) 24 as described later. As a result, the domain walls 10 b arepinned to the regions where the nonmagnetic phase(s) 22 exist, as shownin the schematic views in FIGS. 3( a) through 3(c).

FIG. 3( a) shows an example in which a pinning layer having thestructure illustrated in FIG. 2( a) or 2(b) is used as the pinning layer20. The fine particle size of the granular magnetic phases ornonmagnetic phases is in the range of subnanometers to tens ofnanometers. Two or more fine particles of the magnetic phases or thenonmagnetic phases correspond to one record bit. The thickness of thepinning layer 20 is greater than 0 but not greater than 10 nm.

FIG. 3( b) shows an example in which a pinning layer having thestructure illustrated in FIG. 2( e) is used as the pinning layer 20. Thethickness of the pinning layer 20 is greater than 0 but not greater than10 nm. The length of one bit (the distance between adjacent domainwalls) is equal to a length two or more times greater than the distancebetween adjacent nonmagnetic phases 22 sandwiching one magnetic phase 24in the pinning layer 20. In FIG. 3( b), the pinning layer 20 is formedwith stripes of the magnetic phases 24 and the nonmagnetic phases 22arranged at intervals of several nanometers.

FIG. 3( c) shows an example in which a pinning layer having thestructure illustrated in FIG. 2( d) is used as the pinning layer 20. Thepinning layer 20 is formed with stripes of the magnetic phases 24 andthe nonmagnetic phases 22 each having a length of tens of nanometers.The length of one bit (the distance between adjacent domain walls) isequal to the distance between adjacent nonmagnetic phases 22 sandwichingone magnetic phase 24 in the pinning layer 20. The thickness of thepinning layer 20 is greater than 0 but not greater than 10 nm.

The insulating layer 30 is located adjacent to the pinning layer 20. Theinsulating layer 30 can be formed with an insulating material such asaluminum oxide (AlOx), aluminum nitride (AlN), magnesium oxide (MgO),magnesium nitride (Mg—N), silicon oxide (SiO₂), silicon nitride (Si—N),silicon oxynitride (Si—O—N), TiO₂, or Cr₂O₃, or a dielectric materialsuch as barium titanate (BaTiO₃), SrTiO₃, PbTiO₃, or HfO₂.

(Domain Wall Shift Operation)

Next, a domain wall shift operation in the magnetic nanowire 10 isdescribed.

As shown in FIG. 4, when an electric field is applied to the pinninglayer 20 via the insulating layer 30, changes are caused in magneticcharacteristics of the magnetic phases 24 of the pinning layer 20, suchas the magnitude of the magnetic anisotropy, the direction of themagnetic anisotropy, or the intensity of the magnetization. Such changesare due to a magnetostrictive effect of the pinning layer 20 or theelectric field effect of the surface of the pinning layer 20. If amaterial with high dielectric properties is used as the insulating layer30, a particularly advantageous effect can be achieved. If theinsulating layer 30 has high dielectric properties, the lattice isdistorted by the electric field. Therefore, the magnetic phases 24 ofthe pinning layer 20 adjacent to the insulating layer 30 have the sizeor direction of the magnetic anisotropy changed by the magnetostrictiveeffect. Also, since an effective electric field can be made larger byenhancing the dielectric properties, the electric field effect of thesurface of the pinning layer 20 easily develops.

If the magnetic anisotropy of the pinning layer 20 becomes lower due tothe electric field, for example, the difference between the domain wallpotential obtained in a case where the domain walls 10 b of the magneticnanowire 10 are in contact with the nonmagnetic phases 22 and the domainwall potential obtained in a case where the domain walls 10 b exist incontact with the magnetic phases 24 becomes smaller. As a result, theforce pinning the domain walls 10 b becomes smaller, and accordingly,the domain walls 10 b starts moving easily. FIGS. 5( a) and 5(b)schematically show the principle. In a case where no voltage is appliedbetween the magnetic nanowire 10 and the electrode layer 40, the pinningforce acts effectively, and the domain walls 10 b are pinned to theregions of the nonmagnetic phases 22 of the pinning layer 20 (FIG. 5(a)). When a voltage is applied between the magnetic nanowire 10 and theelectrode layer 40, on the other hand, an electric field is applied tothe magnetic phases 24 of the pinning layer 20, and the magnetizationstates of the magnetic phases 24 change, to reduce the pinning force. Asa result, shifting of the domain walls 10 b becomes easier (FIG. 5( b)).The polarity of the voltage applied to the pinning layer 20 is decideddepending on the materials constituting the pinning layer 20.

Thus, the current amount required for a shift operation can be reducedby performing a shift operation on the domain walls 10 b while applyinga voltage between the magnetic nanowire 10 and the electrode layer 40.

The thickness of the pinning layer 20 is preferably not greater than 10nm. If the pinning layer 20 is thicker than that, a sufficient electricfield effect cannot be achieved. Also, if the pinning layer 20 isthicker than the above, the current flow is hindered by the pinninglayer, and the current value during a shift operation becomes larger. Ina case where changes in the electronic state of the surface of thepinning layer 20 caused by an electric field are utilized, the thicknessof the pinning layer 20 is preferably not greater than 5 nm.

FIG. 6 is an example timing chart illustrating a shift operationperformed on the domain walls 10 b. The timing to apply the current (theshift current) Is to the magnetic nanowire 10 so as to move the domainwalls 10 b is when an electric field is being applied to the pinninglayer 20. That is, the shift current Is is flowed at the same time as orafter application of a voltage Vp to the pinning layer 20. It ispreferable to cut off the voltage Vp after cutting off the shift currentIs. To reduce the domain wall shift current, the duration Ts duringwhich the current Is is flowed to the magnetic nanowire 10 is preferablyequal to or shorter than the duration Tp during which an electric fieldis applied to the pinning layer 20. That is, it is preferable to satisfythe condition, Ts≦Tp. The rises and falls in the timing chart shown inFIG. 6 are sharp, but those rises and falls can be gentler depending onthe circuit design. Also, in the timing chart, the applied voltage andthe flowed shift current are constant with respect to time. However, theapplied voltage and the flowed shift current can vary in terms of time,as long as the condition, Ts≦Tp, is satisfied.

(Write Unit)

The write unit 16 is provided to a portion of the magnetic nanowire 10.Data is written by determining the magnetization direction of acorresponding target cell (the cell located at the address at whichwriting is to be performed: TC-w) in the magnetic nanowire 10. As shownin FIGS. 7( a) and 7(b), spin-transfer torque writing can be used by thewrite unit 16. In FIGS. 7( a) and 7(b), a magnetic electrode 16 a isprovided in contact with the magnetic nanowire 10 via an intermediatelayer 16 b. The intermediate layer 16 b can be formed with a nonmagneticmetal layer, a nonmagnetic semiconductor layer, or a tunnel barrierlayer.

The magnetization direction of the magnetic electrode 16 a isperpendicular to the film plane of the intermediate layer 16 b. Itshould be noted that the film plane means the interface between theintermediate layer 16 b and the magnetic electrode 16 a, and is a planeparallel to the extending direction of the magnetic nanowire 10. In awrite operation, electrons are made to flow (in the opposite directionfrom the current flow) between the magnetic nanowire 10 and the magneticelectrode 16 a, and data is written by determining the magnetizationdirection of the target cell TC-w from the direction of the electronflow. FIG. 7( a) shows a case where writing is performed so that themagnetization direction of the target cell TC-w becomes parallel to themagnetization direction of the magnetic electrode 16 a. In this case,electrons flow into the target cell TC-w in the magnetic nanowire 10from the magnetic electrode 16 a via the intermediate layer 16 b. FIG.7( b) shows a case where writing is performed so that the magnetizationdirection of the target cell TC-w becomes antiparallel to themagnetization direction of the magnetic electrode 16 a. In this case,electrons flow into the magnetic electrode 16 a from the target cellTC-w in the magnetic nanowire 10 via the intermediate layer 16 b.

The nonmagnetic metal layer serving as the intermediate layer 16 b ofthe spin-transfer torque write unit 16 can be made of Cu, Ag, Au, Al, oran alloy containing at least one of those elements. The tunnel barrierlayer serving as the intermediate layer 16 b can be made of a materialsuch as aluminum oxide (AlOx), aluminum nitride (AlN), magnesium oxide(MgO), magnesium nitride, silicon oxide (SiO₂), silicon nitride (Si—N),silicon oxynitride (Si—O—N), TiO₂, or Cr₂O₃. Also, a nonmagneticmaterial such as graphite can be used as the material of theintermediate layer 16 b.

As the material of the magnetic electrode 16 a of the spin-transfertorque write unit 16, any of the materials mentioned as examples of thematerial of the magnetic nanowire 10 and the material of the magneticphase(s) 24 of the pinning layer 20 can be used.

As shown in FIGS. 8( a) and 8(b), a write line formed with a metal wire16 c, for example, can be used for the write unit 16. The write line ispositioned at a distance from the magnetic nanowire 10, and isperpendicular to the magnetic nanowire 10. In a write operation, a writecurrent Iw is flowed to the write line 16 c, and the magnetic fieldgenerated by the write current Iw is applied to the target cell TC-wlocated at one end portion of the magnetic nanowire 10. As a result, themagnetization direction of the target cell TC-w is determined, to writedata. FIG. 8( a) shows a case where the write line is a single line.FIG. 8( b) shows a case where the write line is a folded wire, and thewrite current can be halved. Further, in the case illustrated in FIG. 8(b), the size of each target cell can be defined by the gap size of thefolded wire, and accordingly, a machining dimension can beadvantageously set as the cell size.

(Read Unit)

As shown in FIG. 1, the read unit 18 is further provided as a part ofthe magnetic nanowire 10. The magnetization direction of a target cellTC-w having shifted to a corresponding position in the magnetic nanowire10 is read by the read unit 18. As shown in FIG. 9( a), the read unit 18can have a structure in which a magnetic electrode 18 a is provided incontact with the magnetic nanowire 10 via a tunnel barrier layer 18 b,so as to read a signal as a tunnel magnetoresistive effect, for example.The tunnel barrier layer 18 b can be made of a material such as aluminumoxide (AlOx), aluminum nitride (AlN), magnesium oxide (MgO), magnesiumnitride, silicon oxide (SiO₂), silicon nitride (Si—N), siliconoxynitride (Si—O—N), TiO₂, or Cr₂O₃. The magnetic electrode 18 a can bemade of a material that is one of those mentioned as the material of themagnetic nanowire 10 and the material of the magnetic phase(s) 24 of thepinning layer 20.

As shown in FIG. 9( b), in the read unit 18, a detection line 18 c canbe provided at a distance from the magnetic nanowire 10, so that themagnetization direction of the target cell TC-w can be read by using theinduced electromotive force generated in the detection line 18 c whenthe domain walls 10 b shift. Also, as shown in FIG. 9( c), a spin-wavetransmission line 18 d can be provided at a distance from or in contactwith the magnetic nanowire 10, so that the magnetization direction ofthe target cell TC-w can be detected as a spin wave signal.

(Current Introducing Portions)

As shown in FIG. 10, two current introducing portions 60 a and 60 b intowhich the shift current for moving the domain walls 10 b formed in themagnetic nanowire 10 is introduced are further provided at portions ofthe magnetic nanowire 10. A current source (not shown) is connected toone (the current introducing portion 60 a, for example) of the currentintroducing portions. The cross-section taken along the section line A-Adefined in FIG. 10 is shown in FIG. 1.

The current Is supplied from the current source is flowed to themagnetic nanowire 10 via the current introducing portion 60 a, to movethe domain walls 10 b. The shifting direction of the domain walls 10 bis the opposite from the flowing direction of the current Is.

A write operation is performed by the shift current Is moving the targetcell TC-w to the location corresponding to the write unit 16. At thetime of reading, the domain walls 10 b located in front of or behind thetarget cell TC-w are moved across the detection line 18, and reading isperformed by the read unit 18.

As described above, the first embodiment can provide a magnetic memorythat includes a magnetic nanowire with a domain wall pinning site (apinning layer). Furthermore, an increase in current during a shiftoperation caused by an increase in the pinning force caused by theintroduction of the pinning site can be restrained by controlling thepinning force through application of a voltage.

Second Embodiment

FIG. 11 shows a magnetic memory of a domain wall shift type according toa second embodiment. The magnetic memory 1 of the second embodiment isthe same as the magnetic memory 1 of the first embodiment illustrated inFIG. 1, except that the write unit 16 and the read unit 18 are replacedwith a write/read unit 15. This write/read unit 15 is formed bycombining the write unit 16 and the read unit 18 of the first embodimentinto one structure, and serves as both of those units.

The spin-transfer torque writing structure illustrated in FIGS. 7(a) and7(b) and the TMR reading structure illustrated in FIG. 9( a) may be usedso that the write/read unit 15 can serve as both the write unit and theread unit. Alternatively, the magnetic field writing generated by thecurrent illustrated in FIGS. 8( a) and 8(b) and the reading using aninduced electromotive force illustrated in FIG. 9( b) can be used sothat the write/read unit 15 can serve as both the write unit and theread unit.

In the second embodiment, the write unit and the read unit are combinedinto one unit, and accordingly, the area to be involved in writing andreading can be made smaller. Thus, the memory capacity per chip can beincreased.

Like the first embodiment, the second embodiment can provide a magneticmemory that includes a magnetic nanowire with a domain wall pinning site(a pinning layer). Furthermore, an increase in current during a shiftoperation caused by an increase in the pinning force caused by theintroduction of the pinning site can be restrained by controlling thepinning force through application of a voltage.

Third Embodiment

FIG. 12 shows a magnetic memory of a domain wall shifting type accordingto a third embodiment. FIG. 12 is a cross-sectional view of the magneticmemory of the third embodiment. The magnetic memory of the thirdembodiment has a structure in which a trench (also called a hole) 210 isformed in an interlayer insulating film 200 placed on a substrate 100having an integrated circuit (not shown) mounted thereon, and themagnetic nanowire 10, the pinning layer 20, the insulating layer 30, andthe electrode layer 40 of the first embodiment are stacked in this orderon the side surfaces and the bottom surface of the trench 210. That is,each of the magnetic nanowire 10, the pinning layer 20, and theinsulating layer 30 is formed into a U-like shape in the trench 210. Theregion in the trench 210 on the opposite side of the insulating layer 30from the pinning layer 20 is filled with the electrode layer 40. Also, astructure formed by stacking the magnetic nanowire 10, the pinning layer20, the insulating layer 30, and the electrode layer 40 in this order isprovided on the upper surface of the interlayer insulating film 200, andthis structure is connected to each of the magnetic nanowire 10, thepinning layer 20, the insulating layer 30, and the electrode layer 40formed in the trench 210.

Accordingly, the electrode layer 30 is T-shaped, as shown in FIG. 12. Awrite/read unit 15 is provided in a portion of the interlayer insulatingfilm 200 located between the bottom surface of the trench 210 and thesubstrate 100. That is, the magnetic memory of the third embodimentgreatly differs from the magnetic memories of the first and secondembodiments in that the magnetic nanowire 10 is located perpendicularlyto the substrate 100.

Taking into account that the read unit and the write unit are connectedto the integrated circuit (not shown) in the substrate 100, the readunit and the write unit are preferably located on a side of thesubstrate 100 and on a side of the magnetic nanowire 10. Although thewrite/read unit 15 serving as both the write unit and the read unit isprovided in the third embodiment illustrated in FIG. 12, the write unitand the read unit can be provided independently of each other in thevertical type as in the horizontal type illustrated in FIG. 1. Where theread unit and the write unit are independent of each other, therespective operation margins can be more easily secured.

The third embodiment can achieve the same effects as those of the firstand second embodiments. Furthermore, the magnetic nanowire 10 isU-shaped in the third embodiment. Accordingly, the cell area in thehorizontal direction can be made smaller, and a larger capacity can beachieved through an increase in density. Whether the magnetic nanowire10 is U-shaped or V-shaped with an open top portion, the same effect ofthe pinning layer can be achieved.

Next, Examples are described.

Example 1

To examine the functions of a pinning layer 20, simulations based onmicromagnetics were performed. Referring now to FIGS. 13( a) through13(d), the results of those simulations are described.

First, a pinning layer 20 having a thickness of 1 nm is formed on amagnetic nanowire 10 that had a width of 20 nm, a thickness of 5 nm, anda saturation magnetization of 200 emu/cm³. The domain wall energy in acase where nonmagnetic phases 22 exists in the pinning layer 20 (FIG.13( b)) is determined, and the domain wall energy in a case wherenonmagnetic phases 22 did not exist in the pinning layer 20 (FIG. 13(c)) is also determined. The pinning potential energy shown in FIG. 13(a) is determined from the difference between those domain wall energies.Pinning potential energies are determined in cases where the lengthd_(Gap) of the nonmagnetic phases 22 in the pinning layer 20 is 4 nm and6 nm. The pinning potential energies determined in this manner are shownin FIG. 13( d). In FIG. 13( d), the abscissa axis indicates the magneticanisotropy energy Ku of the magnetic phases 24 of each pinning layer 20,and the ordinate axis indicates the pinning potential energy. As shownin FIG. 13( d), the pinning potential energy depends on the intensity ofthe magnetic anisotropy energy Ku of the magnetic phases 24. As thepinning potential energy becomes larger, the domain walls 10 b are morestrongly pinned. It became apparent that, since about 2.5 pico-erg isequivalent to 60 kT (k representing Boltzmann constant, T representingroom temperature), thermal fluctuation resistance as high as that of aHDD (Hard Disk Drive) can be achieved by appropriately choosing themagnetic anisotropy energy Ku, though the pinning layer 20 is as thin as1 nm. Also, the pinning force becomes smaller as the magnetic anisotropyenergy Ku becomes smaller.

The magnetic anisotropy energy Ku is then varied from 1.7×10⁷ erg/cm³ to1.3×10⁷ erg/cm³ by applying an electric field to the pinning layer 20,and the current values required for moving the domain walls 10 b in therespective cases are determined. As a result, the current value requiredfor a shift operation is reduced almost by half. Thus, it is confirmedthat application of an electric field to the pinning layer 20 has theeffect to reduce the current value required for a shift operation.

Example 2

Referring now to FIGS. 14 through 18( d), a method of manufacturing themagnetic memory of the second embodiment is described as Example 2. FIG.14 is a top view of the magnetic memory of this example. Spin-transfertorque writing and TMR reading are used, and the write unit and the readunit are combined into a write/read unit 15. The width of a magneticnanowire 10 is 20 nm. FIGS. 15( a) through 16(b) are cross-sectionalviews illustrating the procedures for manufacturing the magnetic memoryof this example, taken along the section line A-A defined in FIG. 14. Apinning layer 20 has the structure illustrated in FIG. 2( c). FIG. 17 isa top view of the pinning layer 20. As shown in FIG. 17, in this pinninglayer 20, the dot diameter was approximately 10 nm, and the dot pitch is10 nm, so that the size of each 1 bit to be recorded in the magneticnanowire 10 became 20 nm. FIGS. 18( a) through 18(d) are cross-sectionalviews illustrating the procedures for manufacturing the pinning layer 20used in this example.

First, a substrate 100 having the pattern of lower electrode lines forreading/writing (not shown) formed beforehand thereon is prepared, andis placed in an ultrahigh vacuum sputtering apparatus. TbCoFe (30nm)/CoFeB (2 nm) is then formed as a magnetic electrode 15 a equivalentto a TMR pinning layer on the substrate 100 via a buffer layer. Thenumeric values in the parentheses indicate film thicknesses. Further, aninsulating layer 15 b made of MgO (1 nm) is formed on the CoFeB layer(FIG. 15( a)).

A resist (not shown) is then applied, and a resist mask is formed fromthe resist by using a lithography technique. Ion milling is performed byusing this resist mask, to process the magnetic electrode 15 a and theinsulating layer 15 b into shapes of approximately 20 nm×20 nm, and formthe write/read unit 15. After that, the region surrounding thewrite/read unit 15 is filled with an insulator such a SiO₂ film 300, forexample, and the resist mask is removed (FIG. 15( b)).

The substrate 100 is again put into the ultrahigh vacuum sputteringapparatus, and CoFeB (1 nm)/TbFeCo (10 nm) is formed as a magneticnanowire material. A magnetic phase to form a pinning layer is furtherformed on the magnetic nanowire material. The magnetic phase is thenprocessed by the later described DSA technique, to form the pinninglayer 20 (FIG. 15( c)). Until this procedure, the magnetic nanowire 10and the pinning layer 20 may not be in a nanowire state, and may remainsimple solid films.

A resist (not shown) is then applied onto the pinning layer 20, and theresist is exposed and developed in a nanowire state by using an EBlithography apparatus, to form a nanowire resist mask. By using thisnanowire mask, ion milling is performed to process the magnetic nanowire10 and the pinning layer 20 into nanowire forms. The nanowire maskformed on the pinning layer 20 is then removed, to complete the magneticnanowire 10 with the pinning layer 20 located adjacent thereto.

An insulating layer 30 made of HfO is then formed. A resist (not shown)is applied, and the resist is exposed and developed by using an EBlithography apparatus, to form a mask corresponding to the insulatinglayer 30. By using this mask, ion milling is performed to pattern theinsulating layer 30. The mask formed on the insulating layer 30 is thenremoved. An electrode layer material formed with Ta/Au is then formed. Aresist (not shown) is applied onto the electrode layer material film,and the resist is exposed and developed by using the EB lithographyapparatus, to form a mask. By using this mask, ion milling is performedto form an electrode layer 40 for applying an electric field to thepinning layer 20. A nonmagnetic insulating layer is then formed, and themask formed on the electrode layer 40 is removed. In this manner, theregion surrounding the electrode layer 40 is filled with a nonmagneticinsulating layer 310 (FIG. 16( a)).

A resist mask (not shown) covering the electrode layer 40 is then formedby using a lithography technique. By using this resist mask, ion millingis performed to pattern the nonmagnetic insulating layer 310, to formopenings that reach the upper surface of the magnetic nanowire 10. Afterthat, the openings are filled with a metal, to form current introducingvias 58 a and 58 b connecting to the magnetic nanowire 10. A metal filmis then formed, and patterning is performed on the metal film, to formcurrent introducing portions 60 a and 60 b connecting to the currentintroducing vias 58 a and 58 b, respectively.

In the above manner, the magnetic memory 1 that includes the pinninglayer 20 and uses the magnetic nanowire 10 can be manufactured.

Referring now to FIGS. 18( a) through 18(d), formation of the pinninglayer 20 having the magnetic phases 24 formed with dots pitched at 20 nmas shown in FIG. 17 is described. This pinning layer 20 is formed by aDSA technique using polystyrene-polydimethylsiloxane (PS-PDMS).

First, a guide (not shown) formed from a resist having grooves parallelto the magnetic nanowire 10 is formed on a magnetic layer 24 by usingelectron beam lithography. A PS-PDMS solution is then applied to thegrooves. As the PS-PDMS solution to achieve dots pitched at 20 nm, aPS-PDMS solution with a PS molecular weight of 12,000 and a PDMSmolecular weight of approximately 3,000 is used. A thermal treatment isthen performed at 180° C., to cause a microphase separation in thePS-PDMS. As a result, PDMS dots 420 are cyclically formed in the PSmatrix 410, and a PDMS thin layer 430 is formed on the surface layer ofthe PS matrix 410 as shown in FIG. 18( a). The thin layer 430 is removedby CF₄ plasma etching, and etching is further performed through an O₂plasma etching process until the magnetic phase 24 surfaces. As aresult, a mask having the PDMS 420 dots left as a convex pattern isformed (FIG. 18( b)). By using this mask 420, etching is performed onthe extremely thin magnetic phase 24, and a MgO film to be a nonmagneticphase 22 is further deposited (FIG. 18( c)). After that, the maskportions are removed by CF₄ plasma etching and a wet process (FIG. 18(d)). In this manner, the pinning layer 20 can be formed.

Example 3

Referring now to FIGS. 19 and 20, respective timings to write data intoa magnetic nanowire having a pinning layer attached thereto with spintorque, and to read data by a TMR read method are described as Example3. This operation method can be applied to any of the first throughthird embodiments.

(Writing)

FIG. 19 shows an example of a timing chart obtained when spin-transfertorque writing is performed. First, a shift operation is performed. Anelectric field Vp is applied to a pinning layer 20, to weaken thepinning force. A shift current Is is flowed to a magnetic nanowire 10,to move domain walls 10 b. The shift current Is is flowed while thevoltage Vp is being applied to the pinning layer 20. The time durationTs of flowing a current to the magnetic nanowire 10 preferably satisfiesthe relationship, Ts≦Tp, with respect to the time duration Tp ofapplication of the voltage Vp.

In the example illustrated in FIG. 19, writing is performed when thevoltage Vp is zero. This method is particularly effective in a casewhere spin-transfer torque writing is performed by using the magneticnanowire 10 also as the path of a write current Iw at the time ofrecording. In that case, one of the electrodes for applying a current toa TMR read unit is provided below the magnetic electrode 16 a shown inFIGS. 7( a) and 7(b), and the other one of the electrodes is provided inthe electrode (one of the current introducing portions 60 a and 60 bshown in FIG. 10, for example) for flowing a current to the magneticnanowire 10. This structure contributes to a decrease of the number ofelectrodes required for TMR measurement, and accordingly, is desirablein achieving a higher memory density. However, a current is also flowedto the magnetic nanowire 10 at the time of writing, and there is apossibility that the domain walls 10 b are made to shift by the current.To counter this problem, the voltage to be applied to the pinning layer20 is made zero at the timing of writing, to achieve a pinning effect.In this manner, unexpected domain wall shifts by the write current canbe effectively prevented. In other words, the current margin requiredfor the write current and the shift operation current can be effectivelyreduced.

If the magnetic nanowire 10 does not serve as the path of the writecurrent Iw, the electric field can continue to be applied to the pinninglayer 20 throughout a sequence of shift operations and write operations.

Although a shift operation is performed before the start of a writeoperation in the above description, it is possible to perform a writeoperation, a shift operation, a write operation, and a shift operationin this order.

(Reading)

FIG. 20 is a timing chart of TMR reading. First, a shift operation isperformed.

The electric field Vp is applied to the pinning layer 20, to weaken thepinning force. The shift current Is is flowed to the magnetic nanowire10, to move the domain walls 10 b. The shift current Is is flowed whilethe voltage Vp is being applied to the pinning layer 20. The timeduration Ts of flowing a current to the magnetic nanowire 10 preferablysatisfies the relationship, Ts≦Tp, with respect to the time duration Tpof application of the voltage Vp.

In the example illustrated in FIG. 20, a read operation is performedwhen the voltage Vp is zero. This method is particularly effective in acase where TMR reading is performed by using the magnetic nanowire 10also as the current path of the read current Ir at the time of reading.Unexpected domain wall shifts by the read current can be preventedthrough the pinning by the pinning layer 20.

If the magnetic nanowire 10 does not serve as the current path of theread current Ir in TMR reproduction, or in a case of spin wavereproduction, the electric field can continue to be applied to thepinning layer 20 throughout a sequence of shift operations and readoperations.

Although a shift operation is performed before the start of a readoperation in the above description, it is possible to perform a readoperation, a shift operation, and a read operation in this order.

The rises and falls in the timing charts shown in FIGS. 19 and 20 aresharp, but those rises and falls can become gentler depending on thecircuit design. Also, in the timing chart, the applied voltage and theflowed currents are constant with respect to time. However, advantageouseffects can be achieved even if the applied voltage and the flowedcurrents vary, as long as the above described condition is satisfied.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein can be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein can be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

What is claimed is:
 1. A magnetic memory comprising: a magnetic layerincluding a plurality of magnetic domains and a plurality of domainwalls separating the magnetic domains from one another, the magneticlayer extending in a direction; a pinning layer formed with nonmagneticphases and magnetic phases, the pinning layer extending in an extendingdirection of the magnetic layer and being located adjacent to themagnetic layer; an electrode layer located on the opposite side of thepinning layer from the magnetic layer; an insulating layer locatedbetween the pinning layer and the electrode layer; a current introducingunit configured to flow a shift current to the magnetic layer, the shiftcurrent causing the domain walls to shift; a write unit configured towrite information into the magnetic layer; a read unit configured toread information from the magnetic layer; and a voltage generating unitconfigured to generate a voltage to be applied between the pinning layerand the electrode layer.
 2. The memory according to claim 1, wherein themagnetic layer is formed on a substrate having an integrated circuitmounted thereon, and the pinning layer is located on the opposite sideof the magnetic layer from the substrate.
 3. The memory according toclaim 1, wherein a thickness of the pinning layer is greater than 0 nmand not greater than 10 nm.
 4. The memory according to claim 1, whereinthe pinning layer has a structure in which one of the magnetic phasesand one of the nonmagnetic phases are alternately arranged in theextending direction of the magnetic layer.
 5. The memory according toclaim 1, wherein the pinning layer has a striped structure in which oneof the magnetic phases and one of the nonmagnetic phases are alternatelyarranged in the extending direction of the magnetic layer.
 6. The memoryaccording to claim 1, wherein the pinning layer has a structure in whichplural ones of the magnetic phases are arranged in one of thenonmagnetic phases in the extending direction of the magnetic layer. 7.The memory according to claim 1, wherein in the pinning layer, a lengthmultiple times longer than a distance between adjacent ones of thenonmagnetic phases in the extending direction of the magnetic layer isequal to a length of one memory bit, the adjacent ones of thenonmagnetic phases sandwiching one magnetic phase.
 8. The memoryaccording to claim 1, wherein in the pinning layer, a distance betweenadjacent ones of the nonmagnetic phases in the extending direction ofthe magnetic layer is equal to a length of one memory bit, the adjacentones of the nonmagnetic phases sandwiching one magnetic phase.
 9. Thememory according to claim 1, wherein the pinning layer has a granularstructure in which one of the magnetic phases and the nonmagnetic phasesis precipitated into the other one of the magnetic phases and thenonmagnetic phases.
 10. The memory according to claim 1, wherein atleast one nonmagnetic phase is formed with an insulator.
 11. The memoryaccording to claim 1, wherein at least one nonmagnetic phase is formedwith a conductor.
 12. The memory according to claim 1, wherein the writeunit and the read unit are combined into one, and have the samestructure.
 13. The memory according to claim 1, wherein the shiftcurrent is flowed to the magnetic layer from the current introducingunit while the voltage generating unit is applying the voltage betweenthe pinning layer and the electrode layer.